Thin film deposition techniques are widely used in the manufacturing of microfeatures to form a coating on a workpiece that closely conforms to the surface topography. In the context of microelectronic components, for example, the size of the individual components in the devices on a wafer is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) are increasing. The size of such wafers is also increasing to provide more real estate for forming more dies (i.e., chips) on a single wafer. Many fabricators are currently transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.
One widely used thin film deposition technique is chemical vapor deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a solid thin film at the workpiece surface. A common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction.
Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then a high workpiece temperature is needed to achieve a reasonable deposition rate. Such high temperatures are not typically desirable because heating the workpiece can be detrimental to the structures and other materials already formed on the workpiece. Implanted or doped materials, for example, can migrate within silicon workpieces at higher temperatures. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the intended surface of the workpiece. This is undesirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used.
Atomic layer deposition (ALD) is another thin film deposition technique. FIGS. 1A and 1B schematically illustrate the basic operation of ALD processes. Referring to FIG. 1A, a layer of gas molecules A coats the surface of a workpiece W. The layer of A molecules is formed by exposing the workpiece W to a precursor gas containing A molecules, and then purging the chamber with a purge gas to remove excess A molecules. This process can form a monolayer of A molecules on the surface of the workpiece W because the A molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of A molecules is then exposed to another precursor gas containing B molecules. The A molecules react with the B molecules to form an extremely thin layer of solid material C on the workpiece W. The chamber is then purged again with a purge gas to remove excess B molecules.
FIG. 2 illustrates the stages of one cycle for forming a thin solid layer using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor A, (b) purging excess A molecules, (c) exposing the workpiece to the second precursor B, and then (d) purging excess B molecules. The purge process typically comprises introducing a purge gas, which is substantially non-reactive with either precursor, and exhausting the purge gas and excess precursor from the reaction chamber in a pumping step. In actual processing, several cycles are repeated to build a thin film on a workpiece having the desired thickness. For example, each cycle may form a layer having a thickness of approximately 0.5–1.0 Å, and thus it takes approximately 60–120 cycles to form a solid layer having a thickness of approximately 60 Å.
One drawback of ALD processing is that it has a relatively low throughput compared to CVD techniques. For example, ALD processing typically takes several seconds to perform each A-purge-B-purge cycle. This results in a total process time of several minutes to form a single thin layer of only 60 Å. In contrast to ALD processing, CVD techniques only require about one minute to form a 60 Å thick layer. In single-wafer processing chambers, ALD processes can be 500%–2000% longer than corresponding single-wafer CVD processes. The low throughput of existing single-wafer ALD techniques limits the utility of the technology in its current state because ALD may be a bottleneck in the overall manufacturing process.
One promising solution to increase the throughput of ALD processing is processing a plurality of wafers (e.g., 20–250) simultaneously in a batch process. FIG. 3 schematically illustrates a conventional batch ALD reactor 10 having a processing enclosure 20 coupled to a gas supply 30 and a vacuum 40. The processing enclosure 20 generally includes an outer wall 22 and an annular liner 24. A platform 60 seals against the outer wall or some other part of the enclosure 20 via a seal 62 to define a process chamber 25. Gas is introduced from the gas supply 30 to the process chamber 25 by a gas nozzle 32 that introduces gas into the main chamber 28 of the process chamber 25. Under influence of the vacuum 40, the gas introduced via the gas nozzle 32 will flow through the main chamber 28 and outwardly into the annular exhaust 26 to be drawn out with a vacuum 40. A plurality of workpieces W, e.g., semiconductor wafers, may be held in the processing enclosure in a workpiece holder 70. In operation, a heater 50 heats the workpieces W to a desired temperature and the gas supply 30 delivers the first precursor A, the purge gas, and the second precursor B as discussed above in connection with FIG. 2.
However, when depositing material simultaneously on a large number of workpieces in an ALD reactor 10 such as that shown in FIG. 3, it can be difficult to uniformly deposit the precursors A and B across the surface of each of the workpieces W. Removing excess precursor from the spaces between the workpieces W can also be problematic. In an ALD reactor such as that shown in FIG. 3, the primary mechanism for removing residual precursor that is not chemisorbed on the surface of one of the workpieces is diffusion. This is not only a relatively slow process that significantly reduces the throughput of the reactor 10, but it also may not adequately remove residual precursor. As such, conventional batch ALD reactors may have a low throughput and form non-uniform films.